Infrared gas analyzers of the type contemplated by the present invention typically employ an infrared source to pass infrared energy through an unknown gas mixture in a sample cell. Such gas analyzers operate on the principle that various gases exhibit a substantial absorption characteristic at specific respective wavelengths in the infrared radiation spectrum. The energy passing through the sample cell is detected to produce an electrical signal representative thereof. The resulting signal for each gas to be monitored in the analyzer is converted to an output indicating the concentration of the respective gases in the sample cell. Gas analyzers of this type are shown and described respectively in U.S. Pat. No. 4,013,260 issued on Mar. 22, 1977 and in U.S. Pat. No. 4,346,296 issued on Aug. 24, 1982, both assigned to the assignee of the present invention.
Gas analyzers such as those disclosed in the above references employ a beam of infrared energy passing through the sample cell containing an unknown gas mixture, the infrared energy beam being varied by the position of one or more filters in the path of the light beam. Typically, each filter passes only radiation at a characteristic absorption wavelength for a respective gas of interest. One or more additional filters may also be used as reference filters at wavelengths close to the characteristic absorption wave length for any gas present in the sample cell.
A simplified gas analyzer may also use a stationary filter or multiple filters with associated detectors rather than rotary filter wheel as described above. Such analyzers cause an AC signal to be produced by the detector by periodically interrupting the infrared beam, for example with a rotary chopper wheel.
It is known that ambient condition variations such as temperature, pressure, humidity and the like can adversely affect the accuracy of the measurements taken by gas analyzers. Certain inventions have been made to address these concerns. For example, U.S. Pat. No. 4,398,091 issued to Passaro entitled "Temperature Compensated Gas Analyzer" describes a gas analyzer that utilizes various means for compensating for temperature variations that improve the accuracy of the analyzer. Passaro discloses a preamplifier which is coupled to the output of each detector in the analyzer. The preamplifier includes adjustment means for correcting errors resulting from variations in the ambient or operating temperature of the detector. Passaro also teaches means for compensating for variations in the ambient or operating temperature of the sample cell itself. In this regard, an output amplifier within the processing circuit includes an adjustable means to produce offsetting compensation in the output amplifier to correct for the temperature variations in the sample cell.
Apparatus disclosed in the prior art shows the use of more than one gas cell in a gas analyzer. For example, a gas analyzer that has two cells is described in U. S. Pat. No. 3,529,152 in the names of J. P. Strange, et al. entitled, "Infrared Radiation Detection Device for a Non-Dispersive Selective Infrared Gas Analysis System." Strange, et al. disclose a non-dispersive gas analyzer wherein a pair of infrared sources produce energy in two separate beams, one of which is sent through a reference cell and the other of which is directed through a sample cell. The infrared energy in each of the beams is modulated and detected by separate detectors which produce output signals representative of the infrared energy passing through each of the cells. These two resulting signals are then processed together to produce an indication of the composition of the gas in the sample cell.
In a gas analyzer of this type, the reference cell contains a gas mixture that includes a known percentage of a particular gas to be analyzed. By comparing the intensity of the infrared energy passing through the gas mixture contained within the reference cell with the intensity of the infrared energy passing through the gas contained within the sample cell, processing electronics can derive the percentage of unknown gas in the sample cell gas mixture with a high degree of accuracy.
Although all of the above-mentioned gas analyzers work effectively for their intended purposes, they all suffer from a common deficiency when analyzing certain types of gas mixtures. More particularly, these analyzers are not as effective when measuring for the presence of gases in a gas mixture when more than one of those gases absorb infrared energy at approximately the same frequencies.
To more fully explain the problems encountered when measuring a plurality of gases, the following discussion will be directed toward the measurement for the presence of a plurality of gases in the exhaust of an automobile engine. However, a person of ordinary skill in the art will recognize that the principles thereof can be applied to other types of gas mixtures and that application would be within the spirit and scope of the present invention.
In the exhaust gas mixture produced by an automobile engine, gases are measured to determine, for example, the percentage of pollutants that are being expended when the automobile is operating. Typically, the gas mixture is analyzed for the presence of hydrocarbons, carbon monoxide, carbon dioxide and NO.sub.x. In the context of this application what is meant by NO.sub.x are the oxides of nitrogen present in the automobile exhaust. These may include NO, NO.sub.2 and the like.
There is a particular difficulty in measuring the NO.sub.x gas using infrared absorption techniques due to two factors. Firstly, the water vapor (H.sub.2 O) present in typical automotive vehicle emissions absorbs infrared energy at approximately the same frequencies as that of the NO.sub.x gas. Since, water is a much stronger absorber of infrared energy than NO.sub.x, its presence interferes with the accuracy of the measurement of NO.sub.x Secondly, because carbon dioxide (CO.sub.2) also absorbs infrared energy at approximately the same frequency as NO.sub.x gas, its presence in the exhaust gas also interferes with the measurement of NO.sub.x.
It is known that if the gas mixture entering the sample cell is dehumidified, typically by chilling the gas, a substantial part of the water vapor resident therein can be condensed out of the sample cell. However, such dehumidification of the sample cell will also condense out some of the heavier hydrocarbons present in the gas mixture. Hence, the measurement for the presence of hydrocarbons within the same cell would be inaccurate if the gas mixture entering the cell is dehumidified.
Therefore, there is a need for an infrared gas analyzer that can accurately measure for the presence of plurality of gases where the measurement of one gas is impeded by the presence of one or more other gases in the mixture. More particularly, there is a need for an infrared gas analyzer that can accurately measure for the presence of both NO.sub.x and hydrocarbons from the exhaust of an automobile engine.
It is an object of the present invention to provide an improved non-dispersive gas analyzer.
It is a further object of the present invention to provide a gas analyzer capable of measuring accurately the concentration levels of a plurality of gases within a gas mixture.
It is also an object of the present invention to provide a gas analyzer which can accurately measure the concentration level of both NO.sub.x and hydrocarbons from the exhaust of an automobile engine.